1. Field of the Invention
The present invention relates to a moisture-sensitive device and, more particularly, to a moisture-sensitive device which has a detector element and a reference element and which determines the absolute humidity from the difference between the outputs from these two elements.
2. Description of the Prior Art
Various kinds of humidity sensors have been heretofore developed. Some of them detect relative humidities of the atmosphere, while the others detect absolute humidities. Humidity sensors which detect relative humidities are superior with respect to certain factors, such as detection range, sensitivity, accuracy, response speed, reliability, and environmental resistance, but inferior in the other factors. Where such a humidity sensor is placed within a microwave oven that is being operated, the temperature of the atmosphere surrounding the sensor changes rapidly. Under these conditions, when very slight changes in the amount of water vapor are detected, the relative humidity that is a function of temperature is considered as follows.
It is assumed that the amount of water vapor contained in the detected atmosphere is constant and that only the temperature of the atmosphere increases. Event if the amount of water vapor is constant, the relative humidity drops because the saturated water vapor pressure changes. If temperature is elevated at a higher rate, then a very slight increase in the amount of water vapor will not change the relative humidity because of the increase in temperature, or it might lead to a decrease in the relative humidity. Consequently, the result obtained does not substantially change in the amount of water vapor. In this way, this kind of humidity sensor has a serious problem. Hence, a sensor which can directly detect the amount of water vapor is better adapted to measure the humidity of the aforementioned environment, rather than a sensor detecting the relative humidity.
In order to detect absolute humidities or the amount of water vapor, instruments making use of attenuation of microwaves due to water vapor or absorption of infrared radiation have been employed. Since these instruments can directly detect the amount of water vapor by physical means, they are adapted for detection of slight changes in the amount of water vapor even in an environment involving a rapid temperature change as described above. However, these instruments which includes means for providing temperature compensation are large-scale and considerably expensive to fabricate.
In view of the foregoing, the present applicant and others have developed various small-sized absolute humidity sensors which are excellent in environmental resistance and weatherproofness, and are fabricated using micromachining techniques, as disclosed in Japanese Patent Laid-Open Nos. 145,954/1988, 201,559/1988, 243,817/1988, and 263,426/1988. A typical absolute humidity sensor is shown in FIG. 9, where insulating layers 2 are formed on the surface of a substrate 1. A bridge 3c made of an insulating thin film is formed over one insulating layer 2. A heating element 15 made of a thin film of a thermistor material is formed on the bridge 3c. The heat capacity of this device is very small. The insulating thin film can assume the form of a cantilever or a diaphragm, as well as the form of a bridge. The thermistor material can be SiC, TaN, Ge, or other material. An absolute humidity sensor using a thin-film heating element made from such a thermistor material shows high response speed and high sensitivity,, but the thin film is not stable over a long period of time. In particular, it is not stable against heat. Further, the device is not easy to manufacture, because it is necessary to form electrodes 16, and for other reasons. In a proposed structure of the absolute humidity sensor using a thermistor material as described above, a detector element and a reference element are formed on a single substrate to provide temperature compensation. The reference element is hermetically sealed. However, this humidity sensor also suffers from the above-described drawbacks, since it uses a thermistor material.
As a kind of temperature sensor, a flow sensor using a heating element made of a thin film of platinum is known. The heating element of this sensor is shaped like a bridge in the same way as the device described in connection with FIG. 9. This flow sensor can operate from room temperature to 50.degree. C. or below. The heating element is exposed to the surrounding environment so that it may be directly subjected to flow of the environment.